Light Emitting Diodes (LEDs) are widely used in applications such as Liquid Crystal Display (LCD) back lighting, commercial-freezer lighting, white light illumination, etc. LEDs are typically available in through-hole and Surface Mount Technology (SMT) packages. Through-hole packages are ideal for wave solder board applications. Such through-hole LEDs are typically manufactured with a leadframe having two leads. SMT packages are best used with reflow assembly. SMT devices are also useful when package size constraints are an issue. SMT devices are typically manufactured with a leadframe, or a Printed Circuit Board (PCB) or ceramics substrate.
Some applications, such as stage lighting and automotive lighting, often call for spot lighting or narrow viewing angle light—for example, the desired area of illumination may be 5–20 meters away from the light source and the area to be illuminated may be 1–3 meters in diameter (or width). The divergence of a beam can be calculated using the following trigonometric formula:2×H×tan(θ/2),where H represents the distance of the light source to the area of illumination and θ represents the viewing angle. The narrowest viewing angle for currently available LEDs is about 15°. The following table, entitled Beam Size for Various Narrow Angle LED Packages, shows the divergence of light from currently available LEDs with viewing angles of 15°, 30°, and 45°.
TABLEBeam Size for Various Narrow Angle LED PackagesViewing angleBeam size at the followingof LEDdistances (in meters)package5 m10 m20 m30 m15°1.302.605.207.8030°2.685.3610.7216.0845°4.148.2816.5724.85
As is apparent from the table, even LEDs with the narrowest viewing angles may not meet the objectives of some spot lighting applications.
Collimating light from an LED typically decreases (i.e., makes narrower) the effective viewing angle of the LED. Unfortunately, there are several obstacles to collimating light from LEDs. One such obstacle arises from the fact that an LED die does not provide a perfect point source. A typical LED die dimension is about 250 μm×250 μm (10 mils×10 mils) with an active junction area of about 250 μm×250 μm. Light can originate from any point of the active junction area. Accordingly, it is virtually impossible to ensure that light emitted from an LED die is at a focal point of a lens or reflector.
Another obstacle to collimating light from an LED arises from the fact that LED light, though it may appear to be, is not monochromatic. Photons of different wavelengths have differing indices of refraction. Accordingly, light passing from an LED die through a lens is refracted at different angles according to wavelength. The change of index of refraction according to wavelength is known in the art of optics as chromatic dispersion. Chromatic dispersion is the phenomenon that causes the separation of colors in a prism.